Electric discharge apparatus



April 1959 w. E. LARGE ETAL 2,883,583

ELECTRIC DISCHARGE APPARATUS 7 Original Filed April 19, 1954 4 Sheets-Sheet 1 POWER SUPPLY UNIT SLOPE CONTROL UNIT MAGNETIC AMPLIFIER CIRCUIT Rate of Decoy Adjustment Rate of Risa Adjustment April 21, 1959 w. E. LARGE ET AL 2,833,583

ELECTRIC DISCHARGE APPARATUS Original Filed April 19, 1954 4 Sheets-Sheet 2 JIT [TIMING CIRCUIT SEQUENCE TIMER.

255 UU'UL Fig. IB.

April 21, 1959 w. EQLARGE ETAL ELECTRIC DISCHARGE APPARATUS Original Filed April 19, 1954 4 Sheets-Sheet 3 l HTS 5,2433, gas-rs [L] M [-1 349 AL2- 2545 B ATS 35l FigQiC.

United States Patent 2,883,583 ELECTRIC DISCHARGE APIARATUS William E. Large, Clarence, N.Y., and Edward C. Hartwig, Walnut Creek, Califl, assignors to Westinghouse Electric Corporation, East Pittsburgh, Pa., :1 corpora tion of Pennsylvania Original application April 19, 1954, Serial No. 424,061. and this application March 21, 1957, Serial No. 5

8 Claims. (Cl. 315-252) Our invention relates to electric discharge apparatus and has particular relation to apparatus for controlling the supply of current for welding. This application is a division of application Serial No. 424,061, filed April 19, 1954, and assigned to Westinghouse Electric Corporation, and like the parent application concerns itself with precision electric resistance welding of the type discussed in application Serial No. 381,936, filed September 23, 1953, to William E. Large (hereinafter called Large application). The disclosure of the Large application is incorporated in this application by reference.

In the Large application, apparatus is disclosed for determining the waveform of current pulses supplied for welding and particularly for the welding of such material as aluminum and magnesium and their alloys. The welding pulses which are produced with the apparatus disclosed in the Large application are of composite waveform having a rise component, a weld component and a decay component. The apparatus disclosed in the Large application includes facilities for producing such composite pulses and for giving them any reasonable desired form.

While we have found apparatus disclosed by Large to operate satisfactorily, this apparatus is moderately complex, particularly in the portion which determines the welding pulse waveform and which includes three double triodes connected in a moderately involved circuit.

It is, accordingly, an object of our invention to provide welding apparatus of relatively simple structure particularly suitable for precision welding.

Another object of our invention is to provide welding apparatus including facilities of relatively simple structure for producing a welding pulse having any reasonable waveform over a wide range of waveforms predetermining the rise component, the weld component and the decay componnet of the pulse.

Our invention arises from the realization that highly precise waveform control of the type described above may be achieved by including in the welding apparatus or in the control circuit for the welding apparatus transducers which are preferably magnetic amplifiers or saturable reactors. The expressions magnetic amplifier and saturable reactor are used here interchangeably to refer to saturable reactor systems either with or without rectifiers connected to the output windings to produce a self-saturating effect. These reactors are so connected and so controlled as to provide facilities for varying the waveform of the output pulses of the apparatus over a wide range.

In accordance with our invention, we provide welding apparatus including a Power Supply Unit, a Slope Control Unit, a Heat Control Unit and a Sequence Timer in which the Slope Control Unit includes magnetic amplifier or saturable reactor means for determining the waveform of the current supply by the Power Supply Unit. Specifical ly, the saturable reactor means includes output windings, input windings and control windings. Further means is provided for supplying control current to the input windings, which preferably, supplies direct current pulses. The control current supply means and the input windings 'ice of the reactor are connected to the weld timing component of the Sequence Timer in such manner that when the weld timing component is timing out, it selectively sets the control current supplied to the input windings at an appropriate instant in the weld time so as to assure that the current pulse has the desired starting magnitude, the desired maximum magnitude and the desired terminating magnitude. The output windings are connected to the Heat Control Unit and through it control the Power Supply Unit so that the latter supplies current depending on the impedance of the output windings which in turn depends on the control current supplied to the input winding means.

The Slope Control Unit also includes first and second rectifier means and first and second variable impedance means. The first rectifier means and the first variable impedance means are connected in a closed network with one set of control windings, the rectifier means being poled so as to conduct the current induced in these control windings only when the current in the input windings is increased. The second rectifier means and the second variable impedance means are connected in other network with another set of control windings, the second rectifier means being poled to conduct the current which is in duced in the latter control windings only when the current in the input windings is decreased. The decay of current in each of these networks determines the rate of rise of the current through the input windings when it is increased or decreased. Since this decay may be changed by changing the first and second variable impedance means, be...g at a high rate when the impedance means is low and at a low rate when the impedance means is high, the rate of rise and decay of the current in the input windings and the variation of the impedance of the output windings may be correspondingly determined by the magnitude of these variable impedances, and thus the rate of rise and decay of the load current may be set.

The novel magnetic amplifier circuit just disclosed in cooperation with the other components of the welding apparatus thus avails high precision welding apparatus by which the operation of the waveform of the welding pulses may be varied over a wide range.

The novel features that we consider characteristic of our invention are set forth generally above. The invention itself both as to its organization and method of operation, together with the additional objects and advantages thereof, will be understood from the following description of specific embodiments when read in connection with the accompanying drawings, in which:

Figures 1A, 1B and 1C together are circuit diagrams of a preferred embodiment of our invention, and

Figs. 2, 3 and 4 are diagrams showing the essential portions of circuits constituting modifications of our invention.

Description The apparatus shown in Figs. 1A through 1C include a Welder, a Power Supply Unit, a Heat Control Unit, a Slope Control Unit and a Sequence Timer. This apparatus derives alternating current power from a pair of buses or conductors L1 and L2 which may be energized from the available commercial buses (not shown) of a supply of any available voltage such as 230, 440, 550, 2300, or even higher. The Power Supply Unit is supplied directly from the buses L1 and L2. The Heat Control Unit, Slope Control Unit and the Sequence Timer are supplied from auxiliary buses or conductors ALI and AL2 which are energized through a transformer T1 from the main conductors L1 and L2. In addition, the Slope Control Unit includes buses or conductors DL1 and DLZ which supply control current. This control current is of the direct current type and preferably may be aseasss 3 derived through a rectifier (not shown) of suitable structure from the conductors AL1 and AL2 or even directly from the buses L1 and L2.

The welder includes a welding transformer W having a primary P and a secondary S. The secondary S is connected between the welding electrodes E1 and E2. The electrode E1 is movable relative to the electrode E2, and it is actuable by a pressure system shown diagramatically. This pressure system includes a valve V which is closed when the apparatus is quiescent and is actuable by a solenoid SV to open and permit fluid to move the electrode E1 towards electrode E2 so that any work Z may be engaged. The Welder also includes a reservoir having a piston (not shown) for actuating a back pressure switch SP when adequate pressure is built up in the hydraulic system.

The Power Supply Unit includes valve means preferably consisting of a pair of ignitrons 1-1 and 1-2. Each of the ignitrons 1-1 and 1-2 has an anode 5, a cathode 7 and an ignitor 9. The ignitrons 1-1 and 1-2 are connected in anti-parallel between the conductors L1 and L2 and the primary P, the anode 5 of ignitron 1-1 and the cathode 7 of ignitron 1-2 being connected together to one terminal of primary P and the anode 5 of ignitron 1-2 and the cathode 7 of ignitron 1-1 being connected to the conductor L1 While the other terminal of primary P is connected to conductor L2.

A firing thyratron FT1 and FT2, respectively, is associated with each ignitron 1-1 and 1-2. Each firing thyratron FTl and FT2 has an anode 15, a cathode 17 and a control electrode 19. The anodes of the thyratrons FTl and FT2 are respectively connected to the anodes 5 of the ignitrons 1-1 and 1-2. The cathodes 17 of the thyratrons FT1 and FT2 are respectively connected to the ignitors 9 of the ignitrons 1-1 and 1-2.

For controlling the firing of the ignitrons 1-1 and 1-2, a control transformer CT is provided. This transformer has a primary CP and a pair of secondaries CS1 and CS2. The secondaries are connected respectively between the control electrodes 19 and the cathodes 17 of the thyratrons FT1 and FT2 through a suitable bias 21 and 23, respectively, and through grid resistors 31 and 33, respectively. The bias 21 and 23 maintains the thyratrons FT1 and FT2 non-conducting on the quiescent condition of the apparatus. The primary CP is supplied from the Heat Control Unit.

The Heat Control Unit includes thyratrons HCTl, HCT2 and HCT3. Each of these thyratrons has an anode 35, a cathode 37 and a control electrode 39. The thyratrons HCTl, HCT2 and HCT3 are supplied with potential from the conductors AL1 and AL2 through a transformer T2 having a primary P2 connected to the conductors AL1 and AL2 and a secondary S2 having an intermediate tap. The anodes 35 of the thyratrons HCTl and HCT2 are connected to the terminals of the secondary S2. The cathodes 37 of these latter thyratrons are connected together to a common conductor 41, which in turn is connected to the anode 35 of the thyratron HCT3. The cathode 37 of the thyratron HCT3 is connected to the intermediate tap of secondary S2 through the primary CP of transformer CT and through a current limiting resistor 43. There are, thus, in effect two parallel circuits including one or the other of the thyratrons HCTl or HCT2; to these parallel circuits thyratron HCT3 and the primary CP are common. Thyratron HCT3 conducts in series with thyratron HCTl or thyratron HCT2. A resistor R1 is connected between the intermediate tap or the secondary S2 and the conductor 41. This resistor balances the anode voltages impressed on thyratrons HCT1 and HCT2.

The control electrode 39 of thyratron HCT3 is connected to the cathode 37 of this thyratron through a grid resistor 51, a time constant network AN1 and a blocking bias. The network includes a resistor 53 in parallel with a capacitor 55 and has, preferably, a time constant of the order of one period of the potential supplied by the conductors L1 and L2. The bias is adequate to block the conduction of thyratron HCT3 when the network AN1 is uncharged. The network AN1 is supplied from the Sequence Timer through a resistor 57. In the absence of charge on the network AN1, the thyratron HCT3 being non-conducting prevents the conduction of either of the thyratrons HCT1 or HCT2 and thus prevents the current flow through the primary CP. Under these circumstances, the biases 21 and 23 maintain the thyratrons FT1 and FT2 non-conducting.

The control electrode 39 of the thyratron HCT1 and HCT2 are connected to the cathode 37, each through a grid resistor 61 and 63 and another resistor 65 and 67. Between each of the resistors 65 and 67, respectively, and the common conductor 41, firing potential for the thyratrons HCTl and HCT2 is supplied from the Slope Control Unit. This firing potential is effective to fire the thyratrons HCT1 and HCT2 at selected instants in the periods of the supply. The instants being so selected that the proper waveform for the welding current is achieved.

The Slope Control Unit includes 21 Magnetic Amplifier Circuit and a Timing Circuit. The Magnetic Amplifier Circuit includes a pair of magnetic amplifiers MA1 and MAZ. Each amplifier MA1 and MA2 includes a saturable core C1 and C2, an input winding W1-1 and W1-2, an output winding W01 and W02 and a pair of control windings WCl and WC2 and W03 and W04.

The Magnetic Amplifier Circuit also includes a transformer T3 having a primary P3 supplied from the conductors AL1 and AL2 and a secondary S3 having an intermediate tap. The output windings W01 and W02 are connected in series with the secondary S3 through a resistor RP.' The alternating potential from the secondary S3 flows through the windings W01 and W02. The effect of this alternating potential on windings W1-1 and WI-Z must be considered in connecting the latter. The resistor RP has a resistance which is of the same general order as the reactive impedance or inductive reactance of the windings W011 and W02. Thus, the potential impressed between the intermediate tap of the secondary S3 and the junction of the resistor RP and the winding W02 is displaced in phase with reference to the potential between the conductors AL1 and AL2 by an angle depending on the impedance of the windings W01 and W02.

This potential is derived through an output transformer 0T having a primary 0P and a secondary OS. The primary OP is connected between the intermediate tap and the junction of the resistor RP and the winding W02. The secondary OS is connected across the resistors 65 and 67 in the Heat Control Unit. A potential displaced in phase with reference to the anode potentials impressed on the thyratrons HCT1 and HCT2 by a magnitude depending on the impedance of the windings W01 and W02 is thus impressed in the control circuits, respectively, of the thyratrons HCTI and HCT2. This control potential derived from the secondary 0S lags the corresponding anode potential of the thyratrons HCTl and HCT2 in phase by an amount depending on the impedance of the windings W01 andW02. The instant when the thyratrons HCTl and HCT2 are fired is determined by this angle of lag.

The input windings W1-1 and WI-2 are supplied from the conductors DL1 and DL2 through variable resistors RU, RW and RD which may be selectively connected between the buses DL1 and DL2 and the windings W1-1 and WI-2 at different times during the weld interval in accordance with the operation of a plurality of relays REL REZ and RE3. Resistor RU, depending on its magnitude, sets the magnitude at which the rise component of the welding pulse starts; resistor RW similarly sets the magnitude of the weld component of the welding pulse; and resistor RD sets the final magnitude of the decay component of the welding pulse. Relay RE1 has a normany open contact 71; reliayREZ, a normally open con tact '73 and a normally closed contact '77 and relay RE3 a normally open contact 75 and normally closed contacts 4-65 and 407. The resistors RU, RW and RD are adapted to be connected in series with the windings WI-1 and NI-2 through the normally open contacts 71, 83 and 75 of the relays REl, REZ and RE3 depending on the actuation of these relays. The windings WI-l and WI-2 are connected opposing in these series circuits in such manner that any alternating potential induced in them from the output winding W01 and W02, respectively, is balanced out in the series circuit. The coil of relay REl' is connected in series with normally closed contact 77 and normally closed contact 4&7 and the conductors AL1 and AL2. The coil of relay REZ is connected between conductor ALI and a conductor AL3 of the Sequence Timer, which supplied the signal from the weld timing component or the Sequence Timer, the contact 405.

The control windings WCl and WC2 are connected in a closed network with a variable resistor RDU and a rectifier X1. The control windings WC3 and WC4 are connected in series with a variable resistor RDD and a rectifier X2. The windings WC1 and WC2 and WC3 and WC4, respectively, are connected in series opposing so that any potential induced in these windings by the alternating potential flowing through the output windings W01 and W02 is balanced out. Potential is also induced in the windings WC][ and WC2 and WC3 and WC4 when the current flow through the input windings WI-1 and WI-Z changes from a lower magnitude to a higher magnitude as occurs during the rise component of the Welding pulse and from a higher magnitude to a lower magnitude as occurs during the decay component of the welding pulse. These induced potentials are of one polarity during the rise component and of the opposite polarity during the decay component. In each case, these induced potentials are not balanced out in the windings WCl and WC2 and WC3 and WC4 because while these windings are connected opposing, the potential is derived from windings WI-l and WI-2 also connected opposing. Thus, potentials of opposite polarity arising from the change in current flow through windings W14 and WI-2 appear across the windings WC1 and WC2 aiding during the rise component of the Weld impulse and across WC3 and WOi aiding during the decay component of the welding respectively. The rectifier X1 is connected to conduct the current flowing under this induced potential of the polarity occurring during the rise component of the welding pulse, and the rectifier X2 is connected to conduct the current flowing under the induced potential of the polarity occurring during the decay component. The resistors RDU and RDD, respectively, determine the time taken for the current arising from these induced potentials to decay. If these resistors are very low, this time is long, and if they are high, this time is short. rents, in turn, determine the rate of decay in flux in the cores C1 and C2, they determine the rate of rise and decay of the rise components and decay components of the welding pulse.

The impedance of the output winding W01 and W02 of the magnetic amplifiers MAI and MA2 depends on the direct current supplied to the input windings WI-l and WI-2. This, in turn, depends on the setting of the resistors RU, RW, and RD and the operation of the contacts 71, 73 and 75. Thus, the phase angle of the potential derived from the secondary OS which determines the angle of the firing of the thyratron HCTl and HCTZ and which depends on the relationship between the resistors RD and the reactance W01 and W02 is, in turn, dependent on the setting of the various resistors RU, RW and RD and on their connection to the windings WI-l and Wit-2. By properly setting these resistors and by properly determining when they are connected in the circuit, the durations of the rise component and the decay component of a welding pulse may be set. In addition, by

pulse, A

Since these induced curproperly setting the resistors RDU and RDD, the rate of rise of the rise component and the rate of decay of the decay component may be set.

The Timing Circuit includes three thyratrons 1T1, TT2 and TT3. Each of these thyratrons has an anode 85, a cathode 87 and a control electrode 89.

The anode of the thyratron TT1 is connected through a manual switch SW1, which when open serves to turn the Slope Control Unit 011, and through the coil of the relay RE3 to the conductor AL2. The cathode 87 is connected directly to the conductor ALI. A blocking bias is supplied between the control electrode 89 and the cathode 87 by a capacitor 91 which is charged through a rectifier 93 from the secondary HSl of the cathode-heating transformer HTI for the thyratron TT1. This blocking bias in the quiescent condition of the apparatus maintains a thyratron TT1 non-conducting. The blocking bias may be counteracted by potential derived from a transformer T4 having a primary P4 and a secondary S4. The secondary S4 is connected between the control electrode 89 and the cathode 87 of the thyratron TT1 through a grid resistor 95 and the capacitor 91. The primary P4 is connected to the anode 85 0f the thyratron TT2 through a current limiting resistor 101 and to the conductor ALI. The cathode 87 of the thyratron TT2 is connected to the conductor AL2. Current is thus supplied to the primary P4 when thyratron TT2 is conducting. The control electrode 89 of the thyratron TT2 is connected to its cathode 87 through a grid resistor 103 and through a time-constant network SCN. The network SCN includes a capacitor 111 shunted by a fixed resistor 113 and a pair of variable resistors 115 and 117.

The anode 85 of the thyratron TT3 is connected to the conductor AL2 through a current limiting resistor 121 and the network SCN. The cathode 87 of the thyratron TT3 is connected to the conductor ALI. The control electrode 89 of the thyratron TT3 is connected through a grid resistor 123 and the time constant network AN2 to the cathode. The network ANZ includes a capacitor 125 shunted by a resistor 127 and the secondary H82 of the heater transformer HTZ which supplies heating current to the cathode 87 of the thyratron TT3. The secondary HSZ introduces a ripple in the network AN2 which is impressed in the control circuit of the thyratron TT3 displaced in phase with reference to the anode cathode potential of the thyratron. This ripple lags the anode cathode potential in phase and assures that the thyratron TT3 is not fired at the beginning of the positive half-period of the anode cathode potential impressed on it.

The exciting coil of the relay REZ which controls the operation of the Slope Control Unit is supplied with current from the Sequence Timer through the conductor AL3. The coil is connected between this conductor AL3 and the conductor ALI through normally closed contacts 4 35 of relay RE3.

The Sequence Timer includes a squeeze thyratron ST, a weld thyratron WT, a hold thyratron HT and an off thyratron OT. When the operation of the Sequence Timer is initiated, these thyratrons operate in sequence to initiate the squeeze, weld, hold and off intervals of the welding operation. The Sequence Timer also includes a squeeze network SN for timing the squeeze operation, a weld network WN for timing the weld operation, a hold network HN for timing the hold operation and an off network ON for timing the off operation. The Sequence Timer further includes a plurality of auxiliary thyratrons AT1, ATE, AT3, AT4, AT5 and AT6, and a plurality of auxiliary time constant networks ANS, ANQ ANS, AN6 and AN7. In addition, the Sequence Timer includes a transformer T5 having a primary P5 and a plurality of secondaries 155, 285 and 385 and another transformer T6 having a primary P6 having an intermediate tap and a secondary S6. Further, the Sequence Timer includes a series network of a resistor 131 and a capacitor 133 which is connected between the conductors ALll and AL2. The primary P is connected between the junction of the resistor 131 and the capacitor 133 and the intermediate tap of the primary P6. The resistor 131 and the capacitor 133 are of the same order of impedance, and a potential leading the potential between the conductors AL1 and AL2 in phase in accordance with the relationship between the resistor 131 and the capacitor 133 is impressed on the primary PS.

The squeeze thyratron ST has an anode 135, a cathode 137, a first control electrode 139 and a second control electrode 141. The anode 135 is connected to the conductor AL1 through the exciting coil of a starting relay RES. The cathode 137 is connected directly to the conductor AL2. The off network ON includes a timing capacitor 151 shunted by a variable resistor 153 and a fixed resistor 155. The variable resistor 153 may be shunted out by a contact 157 of the repeat-non-repeat switch RNR in the non-repeat position of the switch. The first control electrode 139 is connected to the cathode 137 through a grid resistor 161, another resistor 163, the secondary 385 of the transformer T5 and the off network ON. The resistor 163 and the secondary 385 are shunted by a third resistor 165. The network AN3 includes a capacitor 171 shunted by a pair of fixed resistors 173 and 175 having a center connection so that they operate as a voltage divider and another fixed resistor 177. This network AN3 has a time constant of the order of one period of the supply. The second control electrode 141 is connected to the cathode 147 through a grid resistor 181, a conductor 183 and the network AN3. The control electrode 141 is also connected to the ON network through a resistor 185 which is connected to the conductor 183. A resistor 187 is connected between the anode 13S and the cathode 137. This resistor operates to assure that the coil of the relay RES is deenergized.

The weld thyratron WT includes an anode 195, a cathode 197, a first control electrode 199 and a second control electrode 201. The anode 195 is connected directly to the conductor AL3 and through the coil of the relay REZ and contacts 103 to the conductor AL1. The anode 195 is also connected to the conductor AL1 through the rectifier 203, the resistor 57 and the network ANI. The cathode 197 is connected to conductor AL2, and through rectifier 203 and network AN2.

The rectifier 203 is poled to conduct the current flowing through the weld thyratron WT, but to block current flow in the opposite direction. This rectifier is introduced for the purpose of preventing the coil relay RE2 from discharging networks AN1 and AN2 once they are charged.

When the weld thyratron WT is not conducting, the control electrode 89 of thyratron TT3 is substantially at the potential of the cathode 87 during the half periods when the conductor ALI is negative relative to the conductor ALZ, that is, when the anode 85 of the thyratron TT3 is positive relative to the cathode 87. Thyratron TT3 is then conducting in the absence of conduction of the weld thyratron WT.

The squeeze network SN includes a capacitor 261 shunted by a variable resistor 263 and fixed resistors 265 and 267. A switch is provided for shunting out resistor 265 when the time constant of the network SN is to be low. The squeeze network SN is connected between the contact electrode 199 and the cathode 197 of the weld thyratron WT through the secondary 255, a resistor 270 and the grid resistor 267. The secondary 255 and the resistor 270 are shunted by another resistor 271. The network AN7 includes a capacitor 221 shunted by a resistor 223 and is connected between the control electrode 201 and the cathode 197 of the weld thyratron WT. The network AN7 has a time constant such that the capacitor when charged and permitted to discharge, discharges sufficiently to permit thyratron WT to conduct 8 in a time interval of the order to one period of the supply AL1-AL2. The hold thyratron HT has an anode 375, a cathode 377 and a control electrode 379. The anode 375 of the thyratron ATS is connected to the conductor ALZ through a current limiting resistor 381 and the network AN7. The cathode 377 is directly connected to the conductor AL1. The control electrode 379 of the thyratron ATS is connected through a grid resistor 383, the network AN6 and the network WN to the cathode 377. The control electrode 209 is connected to the cathode 207 through a grid resistor 22S and the network AN7.

The ofI thyratron OT has an anode 385, a cathode 387 and a control electrode 389. The anode is connected to the conductor AL2 through a current limiting resistor 391, a conductor 393 and the ofi network ON. The cathode 387 is directly connected to the conductor AL1. The control electrode 389 is connected to the cathode 387 through the grid resistor 393, another resistor 395, the secondary 185 and the hold network HN. The secondary 185 and the resistor 395 are shunted by another resistor 397.

The thyratron AT1 has an anode 295, a cathode 297 and a control electrode 299. The anode 295 is connected to the conductor AL2 through a current limiting resistor 301 and the network AN3. The cathode is connected directly to the conductor ALI. The network AN4 includes a capacitor 251 shunted by a resistor 253. The control electrode 299 is connected to the cathode through a grid resistor 303 and through the network AN4. The network AN4 has a time constant such that if the capacitor 251 is charged and then permitted to discharge, it discharges sufliciently to permit thyratron AT1 to conduct in about one period of the supply.

The thyratron AT2 has an anode 305, a cathode 307 and a control electrode 309. The anode 305 is connected to the conductor AL2 through a current limiting resistor 311 and the squeeze network SN. The cathode 207 is directly connected to the conductor ALI. The control electrode 309 is connected to the cathode 207 through a grid resistor 321 and the network AN4.

The thyratron AT3 has an anode 235, a cathode 237, a first control electrode 239 and a second control electrode 241. The anode 235 is connected to the conductor AL1 through a current limiting resistor 255 and the network AN4. The cathode 237 of the thyratron AT3 is connected to the conductor AL2.

The first control electrode 239 of the 011 thyratron is connected to its cathode 237 through the contact 281 of the switch RNR when it is in the repeat position, through the grid resistor 161, the network including the secondary 355 and the off network ON. The second control electrode 241 is connected through a grid resistor 285 to the junction of the resistors 173 and and thus to the time constant network AN3. This junction of the resistors 173 and 175 is connected through the pressure switch ASP to the conductor 183. When the pressure switch is closed and the network ANS is charged, the negative potential impressed on the control electrodes 141 of the squeeze thyratron is higher than with the switch open.

The thyratron AT4 has an anode 325, a cathode 327 and a control electrode 329. The network ANS includes a capacitor 331 shunted by a fixed resistor 333 and the secondary H83 of the transformer HT3 which supplied heating current to the cathode heater of thyratron. The anode 325 is connected to the conductor ALI through a current limiting resistor 335 and the network ANS. The cathode is directly connected to the conductor ALZ. The control electrode 329 of thyratron AT3 is connected to its cathode 327 through the grid resistor 269, the network including the secondary 285 and the resistors 270 and 271 and the squeeze network SN.

The thyratron ATS has an anode 345, a cathode 347 and a control electrode 349. The weld network WN includes a capacitor 351 shunted by variable resistors 353 and 355 and a fixed resistor 357. Where a short weld time is desirable, the resistor 355 may be shunted out by a switch 359. The network AN6 includes a capacitor 361 shunted by a portion of the voltage divider 363, a resistor 365 and another resistor 367. The thyratron ATS is supplied with anode potential from the secondary S6. The anode 345 of thyratron AT4 is connected to one terminal of secondary S6 through a current limiting resistor 371. The other terminal of secondary S6 is connected to the cathode 347 of thyratron AT4 through the network WN. The network AN6 is connected across the secondary S6 through the remaining portion of the voltage divider 363. The control electrode 349 of the thyratron AT4 is connected to its cathode through a grid resistor 373 and through the network ANS.

The thyratron AT6 has an anode 205, a cathode 207 and a control electrode 209. The network HN includes a capacitor 211 shunted by a variable resistor 213 and a fixed resistor 215. The anode 205 is connected to the conductor ALl through a current limiting resistor 217 and the network HN. The cathode 207 is connected to the conductor AL2.

The thyratrons ST, WT, AT3, AT4 and AT6 are sup,- plied with cathode heating potential through secondary HS4 of a transformer HT4 and a resistor 401. The heaters of the other thyratrons AT1, AT2, ATS, HT and OT are supplied directly from corresponding secondaries (not shown except for H53) of heater transformers. When the power is first applied to the Sequence Timer, the cathodes of the thyratrons ST, WT, AT3, AT4 and AT6 for this reason reach a temperature at which the thyratrons are capable of conducting about one cycle after the thyratrons AT1, AT2, ATS, HT and OT.

Fig. 1Standby The standby condition of the apparatus is defined as the condition in which the apparatus is ready to produce a weld, but is not in actual operation. In discussing the standby condition of the apparatus, it will be assumed that the switch RNR is set in the repeat position with the contact 281 closed and the contact 157 open as shown. It will also be assumed that the Slope Control Unit is to be in operation and that the switch SW1 is closed as shown.

During the standby condition of the apparatus, the conductors L1, L2, ALI, AL2, DL1 and DL2 are energized. When the supply conductors ALI and AL2 are first energized, the time constant networks AN1 through AN6 and the networks SCN, SN, WN, ON and HN are discharged. Anode-cathode potential is immediately supplied to the thyratrons ST, WT, AT3, AT4 and AT6, but even if this anode potential happens to be positive, these thyratrons fail to conduct for a time interval of the order of one period of the supply because their cathodes do not reach a temperature suflicient for conduction for this interval after the power is applied. Anode-cathode potential is also supplied to thyratrons AT1, ATZ, ATS and HT and OT. Since the heaters of the cathodes of these thyratrons do not include a resistance, these thyratrons are capable of conducting before the others.

Accordingly, thyratrons AT1 and AT2 conduct when their anode-cathode potentials become positive. The conduction of thyratron ATl charges the network AN3 and the conduction of thyratron AT2 charges the squeeze network SN. The charge on the network ANS impresses a small blocking potential between the second control electrode 141 and the cathode 137 of the thyratron ST. The latter then does not conduct when its cathode reaches adequate temperature for conduction, and its anodecathode potential becomes positive. The charging of network ANS also impresses a blocking potential between the control electrode 241 and the cathode 237 of the thyratron AT3 so that the latter is prevented from conducting. The potential ON control electrode 241 is more negative than the potential on control electrode 141 because the former is connected to the iunction of resistors 173 and 175 and the latter to resistor 175 (switch SP being open). Because the squeeze network SN is charged, a blocking potential is impressed between the control electrode 1S9 and the cathode 197 of the weld thyratron WT and also between the control electrode 329 and the cathode 327 of the thyratron AT4. The latter are then also maintained non-conducting. The network ANS is then uncharged and thyratron. ATS conducts. The network Wll is then charged, and it impresses a blocking potential between the control electrode 379 and the cathode 377 of the thyratron HT blocking the latter. The network AN7 is then uncharged and the thyratron AT6, one it can conduct, conducts to charge network HN. Since the control electrode 389 of thyratron OT is connected to network l-lN, this thyratron OT is non-conducting.

Because thyratron WT is non-conducting, the network ANZ is uncharged and thyratron TT3 is conducting. The latter charges the network SCN which impresses a blocking bias between the control electrode 39 and the cathode 87 of the thyratron TT2. The latter is then non-conducting and the transformer T4 is quiescent. The thyratro-n TT1 is then maintained non-conducting by the bias on capacitor 91.

Because the thyratron WT is non-conducting, the relay RE2 is deenergized, its contact 77 is closed and its contact 7 3 is open. Relay RE1 is then actuated through contacts 77 and 497, and its contact 71 is closed connecting the resistor RU in circuit with the input windings WI-l and WI-2 of the magnetic amplifiers MAli and MAZ. The impedance of the output windings W01 and W02. of the magnetic amplifiers MAI and MA2 then depends on the resistance of the resistor RU, and since this resistor is relatively high, the impedance is relatively high. Under these circumstances, the potential delivered by the secondary OS of the transformer OT in the control circuits of the thyratrons HCTI and HCT2 occurs late in the periods of the supply, and if these thyratrons could conduct, they would conduct late in the half periods of the supply and fire the corresponding ignitrons 1-1 and 1-2 late in the half periods so that the low current corresponding to the start of the rise component of the weld impulse would be supplied.

As a matter of fact, the thyratrons HCT1 and HCT2 are not conducting because they must conduct through the thyratron HCT3 and the latter is maintained nonconducting by the bias in the control circuit. This bias can be counteracted by the potential on network AN1, but this network is at this time uncharged because the weld thyratron WT which supplied the charge is non-conducting. Thus, potential is not supplied through the transformer CT to tire the firing thyratrons FTl, and these thyratrons and the ignitrons L1 and 1-2 are non-con ducting.

In addition, the relay RES is deenergized and its contact 403 is open so that the solenoid SV is deenergized and the valve V closed. The electrode V1 is then retracted from the electrode E2 and switch SP is open.

Fig. J-Operati0n To start a Welding operation, the work Z is inserted between the electrodes E1 and E2 and the start switch SS is closed. By the closing of the start switch, the small negative bias impressed on the control electrode 141 of the squeeze thyratron ST by the network ANS is shorted out and the thyratron ST conducts. The conduction of this thyratron causes relay RES to be actuated and contact 403 is closed. Valve solenoid SV is then actuated to open valve V and permit pressure to be applied to move the electrode E1 into engagement with the work. The electrode is moved in this manner and pressure is applied to the work.

The closing of the start switch SS also connects the control electrode 241 of the thyratron AT3 to the cathode 23 through the grid resistor 285 and the resistor 175.

Initially, back pressure switch SP is open and so long as this switch is open, resistor 175, since it conducts discharge current from the capacitor 171, impresses a small blocking bias between the control electrode 241 and the cathode 237 of thyratrons AT3. But when the electrode E1 has engaged the work Z and adequate pressure has been built up, the switch SP is closed connecting the control electrode 241 through the grid resistor 285 to the more positive side of resistor 175. Thyratron AT3 now conducts, locking in the operation so that it proceeds regardless of the position of the start switch SS.

The conduction of thyratron ATS charges network AN4 so that thyratrons ATl and AT2 are rendered non-conducting. When thyratron AT1 stops conducting, the charging of network AN3 is stopped and the latter immediately discharges, assuring that thyratrons ST and AT 3 continue to conduct. When thyratron ATZ stops conducting, the charging of network SN is stopped and this network times out. At the end of the squeeze interval, the network SN has discharged sufficiently to permit thyratrons WT and AT4 to conduct. The conduction of thyratron WT produces a welding pulse, and this phase of the operation will now be considered.

The conduction of thyratron WT causes the network ANT to charge through the rectifier 203 and the resistor 57 in a sense such that the bias in the control circuit of thyratron HCT3 is counteracted, and this thyratron may now conduct. Thyratrons I-ICTl and HCT2 may now also conduct, but the angle at which they are fired is determined by the initial impedance of the windings W01 and W02 and, initially, these thyratrons conduct late in their corresponding half periods of the supply as determined by the resistor RU which is initially connected to supply current to the input windings WI1 and WI-2. During the early parts of the positive half periods of anode-cathode potential supplied to thyratrons HCTl and HCT2, thyratron HCTS is maintained conditioned to conduct by the network ANl. Late in the positive half periods of the potentials supplied to thyratrons HOT]. and

HCTZ when these thyratrons are fired and conduct current through thyratron HCT3, which is still conditioned to conduct, the potential impressed through the transformer CT is adequate to fire the thyratrons FTI and FTZ, and the ignitrons I-1 and I-2, respectively, are correspondingly fired. Small current corresponding to the beginning of the rise component of the welding pulse is now supplied to the primary P of the welding transformer.

The weld thyratron WT also supplies current to the coil of the relay RE2, actuating the latter. The contact 77 is then opened and the contact 73 closed. The opening of contact 77 drops out relay REl which opens contact 71. The current flow through the windings WI-1 and Wl-2 is now determined by the resistor RW which is smaller than the resistor RU.

As the current through the windings WI-l and WI2 increases from the magnitude determined by the resistor RU to the magnitude determined by the resistor RW, potential is induced in the control windings WC1 and WC2 and the decay of this potential is determined by the resistor DRU in series with the rectifier X1. Depending on the magnitude of this resistor, the decay may take place at a low or high rate. Whatever the rate, it determines the rate at which the current flow through the windings W14. and Wl-Z increases from the magnitude corresponding to the resistor RU to the magnitude corresponding to the resistor RW.

As the current flow through the windings WI-l and WI-Z increases, the impedance of the windings W01 and W02 decreases. The phase lag betwen the potential impressed between the anodes and cathodes of the thyratrons HCTl and HCT2 decreases, and the thyratrons,

HCT 1 and HCTZ are fired earlier and earlier in the half periods of the supply. The ignitrons I-1 and I-2 are then also fired gradually earlier in the half periods of correspondingly decreases.

12 the supply, and the current flow through the primary P of the welding transformer W increases from the low initial magnitude to the welding magnitude.

The conduction of thyratron WT also causes the capacitor in the control circuit of thyratron TT3 to charge to such a potential that the thyratron TT3 is rendered non-conducting. The charging of network SCN is interrupted and this network begins to time out.

For the present, there is no change in the operation as i just described. The rise component and the weld component of the welding pulse are produced during this time. At the end of the time interval set by network SCN, the latter times out permitting thyratron 'IT2 to conduct. This thyratron now conducts actuating relay RE3.

The actuation of relay RE?) causes contact 75 to close and contacts 405 and 407 to open. At contact 75 resistor RD is connected in circuit with the input windings WI-l and WI-2 of the magnetic amplifiers MAI and MA2. Relay RE2 is deenergized at contact 403 so that it drops out opening contact 73. At the contact 407, the actuation of relay RE1 is prevented.

The resistor RD is of a higher magnitude than the resistor RW, and the current flow through the windings WI-1 and Wl-Z now tends to decrease. As the current decreases, potential is induced in the windings WC3 and WC4, and current flows through the resistor RDD and the rectifier X2. This current flow determines the rate at which the current flow from the windings WI-l and WI-Z decreases from the magnitude corresponding to resistor RW to the magnitude corresponding to the resistor RD. As this current flow through the windings WI-l and WI-Z decreases at the rate determined by resistor RDD, the reactance of windings W01 and W02 The lag in phase of the potential supplied by the secondary OS with reference to the anode cathode potential of the thyratrons HCTl and HCTZ is not correspondingly increased, the current flow through the ignitrons I-1 and I-Z correspondingly decreased. This current flow continues to decrease until it reaches the magnitude corresponding to constant current how to the windings WI-l and WI-Z, and at this point, the current flow through the primary P has the magnitude at which the decay current of the welding pulse stops.

While the above-described portion of the operation was taking place, the operation of the Sequence Timer progressed. Thyratron AT4 which had been rendered conducting simultaneously with the thyratron WT charged the network ANS. This caused the thyratron ATS to be rendered non-conducting and to permit the weld network WN to time out. The above-described operation of the Slope Control Unit and the Power Supply Unit took place while the network WN was timing out.

At the end of the Weld time, the network WN is sufficiently discharged to permit the hold thyratron HT to conduct. The conduction of the latter charges the network AN7. Network AN7 impresses a bias on the control electrode on the thyratron WT to render this thyratron non-conducting. At this point, the Power Supply Unit and the Slope Control Unit must again be considered.

When thyratron WT becomes non-conducting, the charging of network AN2 is interrupted, and it discharges in a short time interval of the order of one period of the supply sufiiciently to permit thyratron TT3 to become conducting. Network SCN is then charged and renders thyratron TT2 non-conducting. The conduction of thyratron TTl is then stopped and relay RES drops out. Relay R51 is now energized through contacts 77 and 407. Relay RE2 remains deenergized because thyratron WT is non-conducting. The Slope Control Unit is now reset for another operation. In addition, because thyratron WT is non-conducting, the charging of network ANl is stopped and thyratron I- ICT3 becomes non-conducting.

13 The supply of firing pulses by the Heat Control Unit is interrupted and the supply of current to the primary P of the welding transformer W is stopped. Since the squeeze thryratron ST is still conducting, contact 403 remains closed and the valve V remains open so that the electrode E1 remains in engagement with the Work Z.

In the Sequence Timer, the charging of the network AN7 also renders non-conducting the thyratron AT6, stopping the charging of the network HN. This network now times out during the hold time, and at the end of the hold time, permits the off thyratron OT to conduct. The oil thyratron OT now charges the off network ON and blocking potentials are impressed on the control electrode 239 of the thyratron AT3 and on the control electrode 139 of the thyratron ST. Thyratrons ATS and ST are now rendered non-conducting.

Because thyratron ST is non-conducting, relay RES drops out opening contact 403 and closing valve V so that the electrode Ell is retracted from the work. The weld is now completed, and the work Z may be advanced for another weld or another component of work may be inserted between the electrodes.

When thyratron AT3 is rendered non-conducting, network AN4 discharges permitting thyratrons ATI and ATZ to conduct. Networks AN3 and SN are now recharged or reset for another operation. By the charging of network SN, thyratron AT4 is rendered non-conducting, interrupting the charging of network ANS and per mitting thyratron ATS to conduct. Network WN is now charged and reset for a new operation. in addition, thyratron HT is rendered non-conducting, permitting network AN7 to discharge so that thyratron AN6 may again conduct. Network HN is now charged and reset for another operation, and thyratron CT is rendered nonconducting. Network ON may be discharged and be reset for a new operation. The discharge of network ON does not efiect the operation of thyratrons ST and AT3 unless switch SS is closed. If it is, the above-described operation is repeated with the difference that this time thyratrons 151 and ATS are rendered conducting by potentials impressed on their centive electrodes 179 and 239 rather than on the electrodes 141 and 241.

It may be well at this point to consider the effect of transformer T5 and of network ANS on the operation of the apparatus. The secondaries 135, 255 and 355 impress ripples in the control circuits of thyratrons 0T, WT and AT4, and ST and AT3 which lead the corresponding anode potentials in phase thyratrons OT, WT, ST, AT3 and AT4 are thus fired at the beginning of the positive half periods of the supply and the operation is positive. Network ANS includes secondary H83 which introduces a ripple into the control circuit of thyratron ATS which lags the corresponding anode potential in place. This delays the firing of thyratron ATS sulficiently to enable thyratron AT4 to block thyratron ATS during the first half-period during which AT4 is fired. This operation permits the Sequence Timer to time a weld having a duration of one period.

It is seen that with the above-described apparatus a welding impulse suitable for high precision welding having a predetermined rise component, a predetermined weld component and a predetermined decay component is produced. This impulse is produced with apparatus which does not involve the complexity of the apparatus disclosed in the Large application.

Modification In Figs. 2, 3 and 4, the important features of the modifications of our invention are shown. In each case, only the magnetic amplifiers and a few associated components are included.

The modification shown in Fig. 2 shows a magnetic amplifier circuit including magnetic amplifiers MAI and MA2 such as are included in the modification shown in Fig. 1. But in this case the output windings W01 and W02 are connected in parallel in circuit with the phase shifting resistor RP and the secondary S3 of the trans former T3. The impedance of the windings W01 and W02 in this case is equal to the product of the impedances of each of the windings divided by their sum and is substantially smaller than the impedance in the case in which the windings W01 and W02 are connected in series aiding as in the Fig. 1 apparatus.

The modification shown in Fig. 3 is similar to the modification shown in Fig. 2 except that in this case, rectifiers X3 and X4 are connected between the common conductors 4&7 and 409 which lead, respectively, to the secondary S3 and the resistor RP and the output windings W01 and W02. The rectifiers X3 and X4 are oppositely connected so that during the half periods when conductor f-17 is positive relative to conductor 4G9, rectifier X4 conducts through winding W02, and during the other half periods, rectifier X3 conducts through winding W01. The effect of the rectifiers is to produce a self-saturating effect on the cores C1 and C2, respectively, and thus to produce a sharp change in the impedance of the magnetic amplifiers MAI and MA2 in response to a change in the current flow through the input windings WI-l and WI-Z.

The apparatus shown in Fig. 4 includes magnetic amplifiers IMAI and 1MA2, each of which has only three windings, an output winding 1W01 and IWOZ, an input winding 1Wl1 and 1WI-2 and only one control winding IWCl and 1WC2. Across the control windings, the networks for controlling the rate of rise and the rate of decay of the welding pulse are connected in parallel. The former network includes the variable resistor IRDU and the rectifier 1X1, and the other includes the variable resistor IRDD and the rectifier 1X2. The rectifiers 1X1 and 1X2 are oppositely connected, the former conducting the current induced in the control windings iWCl and lWCZ when the current fiow through the input windings lWI-l and 1WI-2 is increased and the latter conducting the current induced in the windings IWCI and 1WC2, when the current in the input windings 1Wl-1 and 1Wl-2 is decreased.

The magnetic amplifier circuits shown in Figs. 2, 3 and 4 are connected to the Sequence Timer, the Timing Circuit, the Heat Control Unit and the Power Supply Unit in the same manner as the Magnetic Amplifier Circuit of the Fig. l embodiment and operate in much the same way.

Conclusion While we have shown and described certain specific embodiments of our invention, many modifications thereof are possible. Our invention, therefore, is not to be limited except insofar as is necessitated by the spirit of the prior art.

We claim as our invention:

1. Apparatus for controlling the supply of Welding current to weld material from an alternating current power supply source including valve means, means for connecting said valve means in circuit with said material to control said current and control means for controlling the magnitude, duration and waveform of said current, the said apparatus being characterized by control means which includes magnetic amplifier means having an output winding means, an input winding means, a first control winding means and a second control winding means, control current supply means, first variable impedance means to set the magnitude of said welding current at the beginning of a weld current impulse, second variable impedance means to set the maximum magnitude of said welding current, third variable impedance means to set the magnitude of said welding current at the end of a weld current impulse, a sequence timer having means for timing a weld time, means actuable by said weld timing means for selectively connecting said first, second and third impedance means in circuit with said input winding means and said control current supply means at appropriate instants during the weld time, fourth variable impedance means for setting the rate of rise of said welding current from said magnitude at the beginning of a weld current impulse to said maximum magnitude, fifth variable impedance means for setting the rate of decay of said welding current from said maximum magnitude to said magnitude at the end of a Weld current impulse, first rectifier means, second rectifier means, means connecting in a closed network said first control winding means said first rectifier means and said fourth impedance means, means connecting in another closed network, said second control winding means, said second rectifier means and said fifth impedance means, and means connecting said output winding means to said valve means, said first rectifier means being poled to conduct the current induced in said first winding means when the control current in said input winding means is increasing and said second rectifier means being poled to conduct the current induced in said second winding means when the control current in said input winding means is decreasing.

2. Apparatus for controlling the supply of welding current to weld material from an alternating current power supply source said apparatus including valve means, means for connecting said valve means in circuit with said material to control said current and control means for controlling the magnitude, duration and waveform of said current, the said apparatus being characterized by control means which includes magnetic amplifier means having an output winding means, an input winding means, and control winding means, control current supply means, first variable impedance means to set the magnitude of said welding current at the beginning of a weld current impulse, second variable impedance means to set the maximum magnitude of said welding current, third variable impedance means to set the magnitude of said Welding current at the end of a weld current impulse, a sequence timer having means for timing a weld time, means actuable by said weld timing means for selectively connecting said first, second and third impedance means in circuit with said input winding means and said control current supply means at appropriate instants during the weld time, fourth variable impedance means for setting the rate of rise of said welding current from said magnitude at the beginning of a weld current impulse to said maximum magnitude, fifth variable impedance means for setting the rate of decay of said Welding current from said maximum magnitude to said magnitude at the end of a weld current impulse, first rectifier means, second rectifier means, means connecting in a closed network said control winding means said first rectifier means and said fourth impedance means, means connecting in another closed network, said control winding means, said second rectifier means and said fifth impedance means, and means connecting said output winding means to said valve means, said first rectifier means being poled to conduct the current induced in said Winding means when the control current in said input winding means is increasing and said second rectifier means being poled to conduct the current induced in said winding means when the control current in said input winding means is decreasing.

3. Apparatus for supplying current for resistance welding including a power supply unit and control means including magnetic amplifier means connected to said unit for controlling said current supplied by said unit in accordance with the magnetization of said magnetic amplifier means, the said apparatus being characterized by that the magnetic amplifier means includes means for determining the rate of change of the magnetization from one magnitude to another thereby to control the rate of change of said current conducted by said unit from one magnitude to another.

4. Apparatus for supplying current for welding material including a power supply unit to be connected to said material for supplying current for welding said material and control means including magnetic amplifier means connected to said unit for controlling said current supplied by said unit in accordance with the magnetization of said magnetic amplifier means, the said apparatus being characterized by that the magnetic amplifier means includes means for determining the rate of change of the magnetization from one magnitude to another thereby to control the rate of change of said current conducted by said unit from one magnitude to another.

5. Apparatus for supplying current for welding material including a power supply unit to be connected to said material for supplying current for welding said material, control means including magnetic amplifier means connected to said unit for controlling said current supplied by said unit in accordance with the magnetization of said magnetic amplifier means and timing means connected to said amplifier means for setting the magnetization of said amplifier means at successive predetermined magnitudes in a predetermined timed sequence to set said current at predetermined magnitudes in a predetermined timed sequence.

6. Apparatus for supplying current for welding material including a power supply unit to be connected to said material for supplying current for welding said material, control means including magnetic amplifier means connected to said unit for controlling said current supplied by said unit in accordance with the magnetization of said magnetic amplifier means and timing means connected to said amplifier means for setting the magnetization of said amplifier means at successive predetermined magnitudes in a predetermined timed sequence to set said current at predetermined magnitudes in a predetermined timed sequence, the said magnetic amplifier means including means for determining the rate of change of the magnetization of said amplifier means for one magnitude to another thereby to control the rate of change of said current from one of said predetermined magnitudes to another during said last-named sequence.

7. Apparatus for supplying current from an alternating current source for welding material including a power supply unit to be connected to said source and to said material for supplying current for welding said material, control means including magnetic amplifier means connected to said unit for setting the instants in each of the half-periods of said source when current fiow from said source during each of said half-periods is initialed in accordance with the magnetization of said magnetic amplifier means thereby to control said current supplied through said unit and timing means connected to said amplifier means for setting the magnetization of said amplifier means at successive predetermined magnitudes in a predetermined timed sequence to set said current at predetermined magnitudes in a predetermined timed sequence.

8. Apparatus for supplying current from an alternating current source for welding material including a power supply unit to be connected to said source and to said material for supplying current for welding said material, control means including magnetic amplifier means connected to said unit for setting the instants in each of the halfperiods of said source when said current flow from said source through said unit during each of said halt-periods is initiated in accordance with the magnetization of said magnetic amplifier means thereby to control said current supplied through said unit, and timing means connected to said amplifier means for setting the magnetization of said amplifier means at successive predetermined magnitudes in a predetermined timed sequence to set said current at predetermined magnitudes in a predetermined timed sequence, the said magnetic amplifier means including means for determining the rate of change of the magnetization of said amplifier means from one magnitude to another thereby to control the rate of change of said current from one of said predetermined magnitudes to another during said last-named sequence.

(References on following page) 17 18 References Cited in the file of this patent 2,697,200 Ball Dec. 14, 1954 UNITED STATES PATENTS fifg et a1 2,225,644 KQPWIIIIann 24, 2:802:146 Van EEQQQIIIIIII: Aug: 6: 1957 2,230,571 Koppelrnann Ian. 28, 1941 5 2,806,142 L r s t. 10, 1951 2,590,582 Stadum et a1 Mar. 25, 1952 a ge 

